The most important thing with the barre chords is the bar, and it's also the hardest to get right. Let's do this in the 5th fret, it's a little easier to do there. Put your thumb about in the middle of the back of the neck, at the 5th fret. Without pushing, or applying any pressure, put your index finger across all the strings. You want to put it as close to the middle of the fret as you can, that way if your index finger is crooked like mine, it should still be between the frets. If you want an idea of the positioning of the fingers, and the thumb. Hold your hand out, and without bending the index finger, tap your thumb with it. This is the best way to grab the neck, it can also help you get used to where your index finger should be.
I think one of the biggest mistakes is, that everyone thinks put the bar down first. I think it works better if you place the thumb first. The position of everyone's thumb will vary though, and it may move some when you put your finger across the strings. This is alright, you're stll trying to find the position for your bar. Keep your index finger as straight as you can, and put it down on the strings. Remember to move it towards your thumb, you should feel the strings pressing against the index finger. You want each string to feel the same, in other words equal pressure, but you still don't press down yet. Now keeping the thumb in place, and keeping your finger straight, lift it off of the strings. Now put the finger back down as before, and then do this again. Tap the strings, as you did your thumb in the previous exercise. This is the position you want to remember, especially where your thumb is.
Now apply the pressure, by pressing down on the strings with your index finger. Play the strings one at a time, and you want each one to be clear. This may take some time to get used to, and your index finger will hurt some until then. Don't over do it, blisters really hurt, and you'll lose practice time while they heal.
When you find a comfortable position for the bar, add the other fingers. Again you want clear notes, when you play the strings. After you feel comfortable with it in the 5th fret, move it to the 7th fret, and then back to the 3rd, then to the 8th. Then you should be ready to tackle the dreaded F, in the 1st fret, next is the 10th fret, then the 12th fret. On an acoustic this is about as far up the neck as you can go, unless it has a cutaway.
I hope this helps everyone, it's the best anology I could come up with to explain this. Once again, don't over do it, and try to give equal time to each position when you practice. Let me know how it goes, and if I can be of any more help.
I also have another option for the A major chord, but I'll have to get some sleep first, lol.
JD